1. Technical Field
This invention generally pertains to a method of optimizing the performance of a variable speed chiller operating at a part load condition, and more specifically pertains to a method of rating the overall performance of such a chiller.
2. Description of Related Art
A typical chiller system includes a closed loop refrigerant circuit comprising a compressor, a condenser, a flow restriction, and an evaporator. Hot, compressed refrigerant discharged from the compressor enters the condenser where the refrigerant is cooled by another fluid, such as ambient air or water from a cooling tower. From the condenser, the compressed refrigerant expands upon passing through the flow restriction, which lowers the refrigerant""s pressure and temperature significantly before the refrigerant enters the evaporator. While inside the evaporator, the refrigerant cools another fluid that is distributed to and circulated through various smaller heat exchangers. The smaller heat exchangers cool various comfort zones, such as rooms or other areas within a building. After passing through the evaporator, the refrigerant returns to the suction side of the compressor to complete the cycle.
The load on a chiller will vary with a change in the cooling demand of the building being cooled and can vary with a change in the temperature of the fluid that cools the condenser. Thus, chillers usually have a way of adjusting its cooling effectiveness to meet the load. To adjust the cooling effectiveness, the refrigerant""s flow rate can be adjusted by varying the compressor""s speed and/or by adjusting the position of the chiller""s inlet guide vanes. Typical inlet guide vanes comprise a set of variable pitch blades that throttle the flow of refrigerant drawn through a suction throat of the compressor. The angular pitch of the blades determines the extent to which the guide vanes restrict the flow.
Examples of a chiller whose cooling effectiveness is adjusted by varying compressor speed and/or the opening of inlet guide vanes are disclosed in U.S. Pat. Nos. 5,355,691 and 4,151,725. The ""725 patent is further representative of U.S. Pat. Nos. 4,282,718; 4,282,719; 4,275,987; 4,355,948, 4,351,160; 4,546,618 and 4,608,833.
Although a chiller can adjust its output to meet various loads, a chiller""s performance rating often only reflects a chiller""s efficiency when operated at full load. For example, a chiller""s efficiency rating may be based solely on its efficiency when operated at full speed and with the guide vanes wide open. Chiller efficiency, as used herein and below, refers to a comparison of a chiller""s power consumption to its cooling effect, and is often expressed in terms of kilowatts per ton.
However, in some cases, a chiller is given a rating that factors in a chiller""s efficiency performance at various loads. Such a rating is known as an IPLV or an integrated part load value whose derivation is explained in ARI Standard 550/590-1998. Even with such a rating, chillers today are still designed for operating at full capacity. In other words, a compressor""s impeller size, motor speed, and other features are chosen to provide maximum efficiency for a given full load condition. Such a design approach places less importance on the chiller""s efficiency at part load. However, in many applications, a chiller may need to run partially loaded more often than fully loaded.
Consequently, there is a need for a chiller optimized for part load conditions while maintaining an ability to function at full load, albeit at reduced efficiency.
An object of the present invention to apply a selection code that optimizes the performance of a variable speed chiller at a part load condition, and rate the chiller""s overall performance based on the chiller""s performance at both full load and part load conditions.
Another object is to have an inverter or variable frequency drive reduce a compressor""s speed at a part load condition, and raise the speed of the compressor at full load, such that the compressor""s full load speed is other than the speed at which it would run if the compressor""s motor were driven at a nominal line frequency of fifty or sixty hertz.
A further object is to produce a chiller that operates more efficiently at part load than at full load.
A still further object is to produce a chiller that operates more efficiently (i.e., lower kw/ton) at part load than at full load by reducing the speed of the compressor and, if necessary, partially closing the inlet guide vanes at the part load condition.
Another object of the invention is provide a chiller with a performance rating that emphasizes a chiller""s higher efficiency (i.e., lower kw/ton) at part load.
Yet, another object is to subject a chiller to a part load condition by reducing the temperature of the fluids that exchange heat with the refrigerant in the chiller""s evaporator or condenser, and then rating the chiller""s performance when operating under such conditions.
These and other objects of the invention are provided by producing a chiller for optimum performance at a part load condition where the chiller""s speed is reduced and the chiller""s inlet guide vanes are partially closed, and providing the chiller with a composite efficiency rating that reflects the chiller""s efficiency (i.e., kw/ton) at full load and part load.
The present invention provides a method of rating a chiller""s performance. The chiller includes a compressor driven by a motor, which in turn is driven by an inverter that creates an electrical output having a variable frequency from an electrical input of a nominal frequency. The method comprises determining a part load value that reflects the chiller""s performance at a part load condition, wherein the variable frequency of the electrical output at the part load condition is at a reduced frequency that is less than the nominal frequency of the electrical input; determining a full load value that reflects the chiller""s performance at a full load condition, wherein the variable frequency of the electrical output at the full load condition deviates from the nominal frequency of the electrical input and is greater than the reduced frequency; and providing a composite rating based on the part load value and the full load value, whereby the composite rating indicates the chiller""s performance overall.
The present invention also provides a method of rating a chiller""s performance, wherein the chiller""s performance is that of a chiller including a compressor adapted to compress a refrigerant whose flow is throttled by an inlet guide vane that can move between a more-open position and a less-open position. The compressor is driven by a motor, which in turn is driven at various speeds by an inverter that creates an electrical output from an electrical input. The electrical output has a variable frequency and the electrical input is at a substantially constant nominal frequency. The method comprises determining a part load value that reflects the chiller""s performance at a part load condition, wherein the variable frequency of the electrical output is at a reduced frequency that is less than the substantially constant nominal frequency of the electrical input and the inlet guide vane is at its less-open position; determining a full load value that reflects the chiller""s performance at a full load condition, wherein the variable frequency of the electrical output is greater than the reduced frequency and the inlet guide vane is at its more-open position; and providing a composite rating based on the part load value and the full load value, whereby the composite rating indicates the chiller""s performance overall.
The present invention further provides a method of rating a chiller""s performance, wherein the chiller""s performance is that of a chiller including a compressor adapted to compress a refrigerant whose flow between an evaporator and a condenser is throttled by an inlet guide vane that can move between a more-open position and a less-open position. The compressor is driven by a motor, which in turn is driven at various speeds by an inverter that creates an electrical output from an electrical input. The electrical output has a variable frequency and the electrical input is at a substantially constant nominal frequency. The method comprises at a full load condition, conveying into the condenser a heat absorbing fluid at a full load condenser temperature, wherein the heat absorbing fluid once inside the condenser absorbs heat from the refrigerant; at the full load condition, conveying into the evaporator a heat emitting fluid at a full load evaporator temperature, wherein the heat emitting fluid once inside the evaporator rejects heat to the refrigerant; at a reduced load condition, conveying into the condenser the heat absorbing fluid at a reduced load condenser temperature that is lower than the full load condenser temperature; at the reduced load condition, conveying into the evaporator the heat emitting fluid at a reduced load evaporator temperature that is lower than the full load evaporator temperature; operating the chiller at the full load condition and at a full load efficiency, wherein the guide vane is at the more-open position, and the variable frequency is at least as great as the substantially constant nominal frequency; operating the chiller at the reduced load condition and at a reduced load efficiency that is better than the full load efficiency, wherein the guide vane is at the less-open position, and the variable frequency is less than the substantially constant nominal frequency; and providing a composite rating based on the full load efficiency and the reduced load efficiency, whereby the composite rating provides an indication of the chiller""s performance overall.
The present invention additionally provides a method of optimizing a variable frequency drive controller controlling the motor of a compressor under a variety of conditions. The method comprises the steps of: determining part load and full load conditions for a particular set of circumstances; selecting a compressor and a motor to optimize their operation at the part load value; and verifying that the selected compressor and the selected motor can be overspeeded to safely operate at the full load conditions. The method can include the further step of repeating the selecting and verifying steps until assured that the overspeeded compressor and motor are always within a predetermined range.